Since a RO membrane reveals a high rejection rate of solute, clear permeate obtained by a RO membrane treatment has a superior water quality and can be reused effectively for a variety of purposes. If, however, water to be treated contains contaminants such as suspended solids and organic substances which foul the RO membrane, there is a problem wherein the RO membrane is contaminated by these contaminants causing declines of flux and rejection rate as the treatment lasts.
In water treatment using a RO membrane, a method continuing a stable operation by averting obstructions such as a decline of flux and increase of operational pressure in a RO membrane apparatus is performed by adopting pretreatment (including coagulation, solid-liquid separation and active carbon treatment) to clarify feed water to the RO membrane, wherein, in order to prevent fouling of the RO membrane and to obtain a high treatment efficiency, the feed water to the RO membrane apparatus is evaluated by a Fouling Index (FI) as set forth in JIS K3802, or Silt Density Index (SDI) as set forth in ASTM D4189, and the pretreatment is carried out wherein the FI or SDI falls in a predetermined value or lower, for instance, the FI or SDI becomes 3-4 or lower.
In a cooling water circulation system, a part of the water evaporates when the cooling water used for cooling a heat source is cooled in a cooling tower, whereby contaminants such as suspended solids and organic substances are condensed. Further, to the cooling water, bacteria which cause slime formation mix from the outer side and slime formed in the cooling tower mix by exfoliating, so that sterilization for slime control becomes necessary. Since, even if contaminants such as suspended solids and organic substances have been removed, bacteria contained in the water to be treated proliferate on the surface of the RO membrane to decrease the flux of the permeate, sterilization of the water to be treated is performed by adding a sterilizer to prevent fouling of the membrane by the proliferation of bacteria.
As a sterilizer for general water systems, a free chlorine agent such as chlorine and sodium hypochlorite is widely used which causes, as an oxidizing agent, the deterioration of the RO membrane and a decline in efficiency. In order to ease the oxidizing effect, Patent Document 1 (JP H01-104310A) proposes the formation of a chloramine by adding ammonium ion after sterilization by a free chlorine agent. However, Patent Document 1 does not disclose the application to water to be treated containing the contaminant in detail, for instance, when and how it is applied.
Patent document 2 (JP2006-263510A) teaches a slime control agent for membrane separation which comprises a combined chlorine agent comprising an oxidizing agent based on chlorine and a sulfamic compound. Patent document 2 also teaches that free chlorine is contained in a specific ratio to the above combined chlorine agent which is added into the water to be treated, and that there is a correlation similar to equilibrium. In order to obtain the sterilizing effect, the above combined chlorine agent is to be used in a concentration wherein free chlorine is detected in the water to be treated, while the combined chlorine agent is used in a concentration wherein the free chlorine concentration is 2-6 mg/L and total chlorine concentration is 20-60 mg/L in the Examples.
Since there are problems that a RO membrane, especially a RO membrane composed of a high molecular weight membrane having a nitrogen-containing group such as a polyamide and aramid is easily damaged by free chlorine and thereby deteriorates the membrane separation efficiency such as the rejection and removing rates, it is important to perform the RO membrane treatment in a condition wherein free chlorine is not present. For this, Patent document 3 (JP H09-57067A) proposes to carry out the RO membrane treatment after sterilizing by a free chlorine agent and then eliminating the sterilizing agent by adding a reducing agent such as sodium bisulfite. In Patent document 3, the copper concentration is restricted because effect of elimination of sterilizing agent by addition of reducing agent is insufficient. It is suggested that elimination of free chlorine after sterilization by free chlorine agent is necessary.
Since, as above, a RO membrane, especially a RO membrane composed of a high molecular weight membrane having a nitrogen-containing group such as a polyamide and aramid is required to be used in a water system containing no free chlorine, residual chlorine should be removed from a feed water sterilized with a chlorine agent before being supplied to a RO membrane. If, however, the feed water is supplied to a RO membrane after the residual chlorine has been removed, slime will form as the treatment lasts to cause the deterioration of the membrane performance. In order to avert these problems, a combined chlorine agent as shown in Patent document 2 can be added in a free chlorine concentration of 0.1 m/L or lower. This method may be possible in the case that the combined chlorine agent is prepared on the spot to be added. However, when a combined chlorine agent prepared in a factory is added, after storage and transportation, on the spot so as to be diluted to a free chlorine concentration of 0.1 m/L or lower, there occurs a problem that the combined chlorine (total chlorine) concentration becomes low to decrease the slime prevention effect. Accordingly, there is a demand for a combined chlorine agent having a low concentration of free chlorine and a high concentration of combined chlorine.